I. Field of the Invention
The present invention relates to the field of analysis of physical and chemical properties of mixtures comprising hydrocarbons, preferably petroleum fuels, by use of Raman spectroscopy, preferably FT-Raman spectroscopy.
The instrument can utilize spectrometers of the Fourier-Transform (FT) Raman variety or of the Dispersive Raman variety or can use Hadamard Transform Raman spectrometers or other spectroscopic techniques known in the art.
Hadamard Transform spectroscopy is described in a paper by Hammaker et al. in Vibrational Spectra and Structure, Vol. 15, November 1986, " . . . The purpose of the spectrum analyzer is to disperse the near infrared radiation passing through the body into its spectral components. Selected wavelength ranges are focused on detector cells, which provide an analog signal proportional to the intensity of radiation in the selected wavelength . . . " (taken from U.S. Pat. No. 5,379,764 to Barnes et. al.)
II. Description of the Prior Art
The Clean Air Act of 1989 has mandated radical change in the petroleum refinery industry. Based on seasonal and geographical considerations, commercial gasoline blends must meet stringent environmental requirements while at the same time providing automotive compatibility and efficiency (Rhodes, A. K. Oil & Gas Journal, 17 Jan. 1994, 16). Conventional methods of determining these characteristics of a fuel are time consuming and expensive. Examples include determining total aromatics and olefins via gas chromatography; determining octane numbers via ASTM knock engine methods; and determining vapor pressure via the Grabner method. Legislators realize the need to improve efficiency and lower costs for these measurements and accordingly allow refineries to use alternative methods which are not approved by the EPA if the alternative methods are accepted by the industry.
As early as 1950, Raman spectroscopy was proposed as a method to determine aromatics and olefins in hydrocarbon mixtures (Heigl, J. J.; Black, J. F.; Dudenbostel, B. F.; U.S. Pat. No. 2,527,122, 24 Oct. 1950). However, until recently, extensive use of Raman spectroscopy in the characterization of hydrocarbons has not been practical. One early limitation to Raman analysis was the absence of a high intensity and stable excitation source. This problem has been overcome with the advent of lasers. Another limitation was the presence of fluorescence in hydrocarbon fuels when excited by visible lasers. The development of Fourier-Transform Raman spectrometers, however, now allows Raman spectra to be collected using NIR lasers (e.g. the Nd:YAG laser emitting at 1064 nm) which eliminate or severely reduce fluorescence in petroleum fuels.
Recently, Raman spectroscopy has been demonstrated as a viable quantitative technique in the analysis of analytes which are present in liquid mixtures as minor components (Shope, R.; Vickers, T. J.; Mann, C. K., 42, Appl. Spectrosc., 1988, 468). Chung, Clarke and others have demonstrated that Raman spectroscopy can be used in the qualitative analysis of aviation fuel for the determination of general hydrocarbon makeup, aromatic components, and additives (Chung, W. M.; Wang, Q.; Sezerman, U.; Clarke, R. H., 45, Appl. Spectrosc., 1991, 1527; Clarke, R. H.; Chung, W. M.; Wang, Q.; DeJesus, S.; Sezerman U., 22, J of Raman Spectrosc., 1991, 79). Williams and coworkers have shown that FT-Raman spectroscopy in combination with chemometrics can be used to determine gas-oil cetane number and cetane index (Williams, K. P. J.; Aries, R. E.; Cutler, D. J.; Lidiard, D. P., 62, Anal. Chem., 1990, 2553). In addition, Seasholtz et. al. have demonstrated quantitative analysis of the percentage of each fuel in fuel mixtures containing three unleaded gasolines (Seasholtz, M. B.; Archibald, D. D.; Lorber, A.; Kowalski, B. R., 43, Appl. Spectrosc., 1989, 1067). Despite these investigations, Raman spectroscopy is still not significantly utilized in the industrial analysis of petroleum fuels.
In contrast, NIR absorbance.backslash.reflectance spectroscopy has gained wide acceptance in the industrial analysis of octane number during the blending process (S. M. Maggard, U.S. Pat. No. 5,349,188, 9 Apr. 1990; S. M. Maggard, U.S. Pat. No. 4,963,745, 16 Oct. 1990). Multivariate analysis of NIR spectra currently provides real-time feedback for on-line process control of blending operations (as well as other processes) at a number of refineries, including the Ashland Petroleum refineries in Catlettsburg, Kentucky, and St. Paul, Minn. Despite the success of NIR spectroscopy in the petroleum industry, NIR also has certain limitations. For example, the overtone absorbances which constitute a NIR spectrum are typically broad and ill-resolved. This results in a decrease in the "chemical information" contained in the spectral data. Applicants have recently shown that fiber-optic Raman spectroscopy with partial least squares analysis is capable of quantifying individual octane numbers and RVP (with standard errors &lt;0.5% vol) in hydrocarbon blends. This advantage over NIR spectroscopy is due to the abundant, yet sharp and well resolved, spectral peaks in the Raman spectra.
Applicants herein describe the use of FT-Raman spectroscopy and the preferred partial least squares (PLS) regression analysis to accurately determine the research octane number (RON), the motor octane number (MON), the pump octane number (PUMP), and the Reid Vapor Pressure (RVP) of 208 commercial petroleum fuel blends produced by the Ashland Petroleum Company.
Kelly et al. used a NIR instrument equipped with fiber-optics to gather spectra for predicting octane after multivariate treatment. See F. X. Garcia, L. D. Lima, and J. C. Medina, 47, Appl. Spectrosc., 1036 (1993). Williams et al. have shown that NIR FT-Raman spectroscopy combined with multivariate statistics can be used to determine the gas oil cetane number and cetane index. See J. B. Cooper, K. L. Wise, J. Groves, and W. T. Welch, Anal. Chem., 16 (22), Nov. 15, 1995. Garcia et al. used mid-IR absorption spectroscopy and partial least squares regression analysis to model percent oxygenates in fuel samples. See J. B. Cooper, K. L. Wise, W. T. Welch, R. R. Bledsoe and M. B. Sumner, Appl. Spectrosc. 50 (7), July 1996. Fiber-optic NIR reflecto-absorbance spectroscopy in tandem with multiple linear regression is used at Ashland Petroleum to monitor the concentration of aromatics and octane number in real time. Applicants have also recently demonstrated that FT-Raman and PLS regression analysis can be used to predict oxygenate concentrations, octane numbers, and Reid vapor pressure in commercial gasolines with a degree of accuracy similar to NIR methods. See 1988 Annual Book of ASTM Standards, Vol. 05.04.